Pressurized water reactors (or PWR reactors) include a vessel and inside the vessel, a nuclear reactor core consisting of fuel assemblies and pieces of internal equipment.
The pieces of internal equipment support the fuel assemblies in the core, ensuring their position and allowing control of the nuclear reaction and extraction of heat.
The pieces of internal equipment comprise upper pieces of internal equipment located above the core and lower pieces of internal equipment located below the core.
The upper pieces of internal equipment include an upper core plate in contact on the upper ends of the fuel assemblies, a supporting plate positioned above the upper core plate while being spaced apart from the latter, and spacer columns extending vertically between the upper core plate and the supporting plate.
The upper pieces of internal equipment comprise cluster guides for guiding the control clusters comprising absorbing pencils containing a neutron absorbing material. The cluster guides extend vertically between the upper core plate and the supporting plate and extend above the supporting plate. In order to control the nuclear reaction, the control clusters are moved vertically so that their absorbing pencils are introduced over a more or less substantial height inside certain fuel assemblies.
The upper pieces of internal equipment comprise conduits for guiding the insertion of thermocouples inside the vessel in order to measure the temperature of the water coolant at the outlet of an assembly of pre-selected fuel assemblies.